Broadcast systems have embraced the demand for high quality transmissions made possible by digital technology. The digital revolution has transformed the delivery of broadband services, including audio and video programming as well as data transmission. Satellite communication systems have emerged as a viable solution for supporting such broadband services. As such, power and bandwidth efficient modulation and coding are highly desirable for satellite communications systems to provide reliable communication across noisy communication channels. In broadcast applications supported by such systems, rapid frame synchronization in low signal-to-noise (SNR) environments is necessary to avoid negatively impacting user experience, as well as utilizing system resources efficiently.
Traditionally, frame synchronization has not been an area of major concern for conventional broadcast and/or continuous transmission systems employing convolutional code, largely because decoding can be performed prior to frame synchronization. Consequently, the post decoding frame synchronization can benefit from the coding gain offered by the error correction codes. For instance, the Digital Video Broadcasting via Satellite (DVB-S) standard has been widely adopted worldwide to provide, for example, digital satellite television programming. Traditional DVB compliant systems employ fixed modulation and coding schemes. At present, such DVB compliant systems utilize Quadrature Phase Shift Keying (QPSK) modulation and concatenated convolutional code and Reed-Solomon channel coding. Given the fact that modulation and coding schemes are fixed, and the fact that the continuous transmission nature of broadcasting or unicasting, a simple framing structure can be utilized for these applications. In actuality, the only framing overhead is a Synchronization (“SYNC”) byte attached to a MPEG 2 (Moving Pictures Experts Group-2) frame. The SYNC byte is treated the same as other data by the convolutional code and the Reed-Solomon encoder. At the receiving end, the data corrupted by the communication media are first recovered by the convolutional code. The convolutional code can function without the knowledge of the framing structure. The output of the convolutional code is of high fidelity, typically at bit error rate below 1×10−5. With the high fidelity output, simple data matching with the SYNC byte is able to identify the starting point of the MPEG frame. Therefore, the transmitted data can be properly reassembled to deliver to the next layer.
However, with block coded systems, frame synchronization is typically achieved before decoding. This is required particularly when the receiver has to determine which modulation and coding is used among a vast amount of potential combinations of modulation and coding schemes. Modern error correction coding, such as low density parity check (LDPC) codes, operates at extremely low signal to noise ratios. This implies that such frame synchronization needs to be achieved at the same low signal-to-noise ratios (S/N or SNR). Furthermore, frame synchronization in such systems extends beyond determining the beginning and ending points of a frame, to determining the modulation and coding scheme employed in the frame.
In view of the foregoing, the conventional approaches to frame synchronization do not operate well in that the requirements of high fidelity outputs, for example, can no longer be guaranteed.
Consequently, other approaches have been developed, but require incurring significant overhead (i.e., reduction in throughput) and receiver complexity. For example, one approach suggests using a forward error correction coding, such as a Bose Chaudhuri Hocquenghem (BCH) code, to protect the framing information within the frame structure. At the receiving end, the receiver searches for the unique word first by correlation. Once the unique word is detected, the BCH coded framing information is decoded coherently by a maximum likelihood correlation decoding. A drawback of this technique is that the unique word has to be large (i.e., high overhead). Another drawback is that true maximum likelihood decoding of the BCH code is quite complex.
Therefore, there is a need for a frame synchronization mechanism that provides rapid acquisition without incurring large overhead costs. There is also a need for a frame synchronization approach that is simple to implement. There is also a need to provide a synchronization technique that is flexible as to provide coding and modulation independence.